1. Field of the Invention
The invention relates to the field of environmental chambers and in particular to an apparatus and method for thermal stress screening of electronic circuitry and assemblies.
2. Description of the Prior Art
The design of environmental chambers has previously not been subject to radical changes from year to year due to the reasonably well fixed or established purposes for which such chambers are used. However, currently significant modifications have been made in these purposes. For example, the environmental changes to which circuits and to which circuit assemblies are subjected in increasingly complex applications in aerospace, near space or outer space, have placed new demands upon reliability as well as environmental cycling which such circuits must withstand. Environmental chambers must create similar environmental stresses on the earth's surface as these circuits will encounter in their ultimate applications In particular there is a growing demand for environmental chambers capable of producing faster thermal changes, for customized designs for unique configurations of chambers for stress screening, and for microprocessor based programmable control of such chambers
In addition, there is a recognized need for ease of maintenance and repair of the environmental chambers Many electronic hardware markets are moving toward 100% testing in some form of temperature cycling of their PC boards, assemblies and final products Failure of the environmental control testing equipment thus becomes a critical manufacturing blockage. Downtime of the environmental test equipment is then translated directly into lost production output.
Furthermore, the accuracy or reliability of the environmental testing itself must be insured if the test certification is to have any validity. In other words, if a circuit assembly is certified as operable in a temperature range even under specified thermal stresses, the test procedure must reliably and actually cycle the circuit assembly across the specified range and at the specified rate. Prior art environmental chambers were controlled so that the chamber environment was cycled through the specified ranges and rates, but it could not be certified that the tested object or circuit assembly actually achieved the specifications. A survey of test procedures and of specific environmental apparatus is set forth generally in a review article entitled "Environmental Screening/Chambers-Faster Cycling, Smarter Controls Typify New Chamber Designs", Evaluation Engineering, May 1985.
What is needed, then, is an apparatus and method which overcomes each of these defects of the prior art. More specifically, what is needed is a temperature control system which can be easily and quickly customized for unique configurations, quickly and easily repaired with minimal downtime, programmably controlled, and which reliably and accurately cycles the tested object through specification ranges and rates.